Thermal and mechanical properties of amorphous selenium films in the glass transformation region

Abstract

Thermal and mechanical properties of photoreceptor-type amorphous selenium (a-Se) films have been studied via differential scanning calorimetry (DSC), thermomechanical analysis (TMA) as well as thermomicrohardness analysis (T mu HA). The a-Se films were vacuum deposited by thermal evaporation onto heated Al substrates and identically aged. The experiments were carried out over a temperature range encompassing the glass transformation to examine the nature of physical processes controlling the thermal and mechanical properties. In DSC experiments, the glass transformation kinetics was investigated via both heating and cooling schedules. By applying the present thermoanalytical methods-Ritland and Kissinger analyses-it proved possible to identify a typical Vogel-Tammann-Fulcher ( approximately exp(A/(T-T0))) type of response in the structural retardation times controlling the relaxation of the enthalpy. It was found that over a common temperature range, the temperature dependence of the mean relaxation time tau (T), whether determined from heating or cooling schedules, followed the viscosity-temperature data for a-Se suggesting that tau (T) approximately eta (T). The temperature dependence of the microhardness, Hv(T), on a semilogarithmic plot evinced a sharp decrease at a temperature Tg in the glass transformation region. The origin of this sharp decrease in the microhardness has been attributed to a structural relaxation process which has a mean retardation time proportional to the viscosity as in the enthalpy relaxation process. In the case of TMA however, it was found that the shift in the softening temperature with the heating rate indicated an apparent activation energy less than that for the viscosity. Two possible models are discussed for the interpretation of the TMA softening temperature which are based on the relaxation of the elastic modulus and an elastic deformation to viscous flow transition. By comparing the results from the present thermal and mechanical studies with the reported viscosity-temperature data, it is concluded that the rate of relaxation of enthalpy and microhardness of a-Se in the glass transformation region is inversely proportional to the melt viscosity which can be adequately described over a temperature range above approximately 30 degrees C by a Vogel-Tammann-Fulcher expression.